Anyway, since I'm doing this in a 5.5x8.5 inch, first I had to set the document size to that, input the margins I wanted, make sure it was applied to ALL the document and not just one section, then go into Page Setup, Manage Custom Sizes and build and name a new setting.
Click the + to add a new setting, input the size you want -- like 5.5 by 8.5 -- then in the Non-Printable area make sure it's User Defined and everything is set to 0. Finally, click OK. Then open your printer and set the Default Paper Size to the same size paper. If you don't, it will put your 5.5x8.5 inch document into the middle of an 8.5x11 size PDF page.
Then you Save to PDF...and it comes out looking exactly like you want...after a half-dozen false starts. Except...for some reason mine's been saved in 2 parts -- the first 12 pages (being section 1) and the rest of the document. That isn't such a big deal for me; I can use the shop's Adobe setup to combine them...but I won't have that forever so I need to find a way around it.
Still...now I can sort of do it and it looks good.